Electrical heating unit



Dec. 10, 1957 w. A. MUDGE ELECTRICAL HEATING UNIT Filed Dec. l5, 1954 it rates Patent OC@ 2,816,200 Patented Dec. 10, 1957 ELECTRECAL HEATING UNIT William Alvin Mudge, New York, N. Y., assigner to The International Nickel Company, inc., New York, N. Y., a corporation ol Delaware Application .December 15, 1954, Serial No. 475,460

3 tlaims. (Cl. 201-67) The present invention relates to electrical heating units and, more particularly, to new and improved electrical heating units of the sheathed type capable of operating at high temperatures for long periods of time and to processes for the production of such heating units.

The art has long sought to produce an electrical heating unit, particularly of the sheathed type, capable of withstanding elevated temperatures of the order of above about 1600 F. for substantial periods of time. The sheathed type of unit may be generally described as cornprising an electrical resistance element embedded in or enveloped by a mass of insulating material, the said resistance element and insulating material being enclosed within a metallic or other suitable sheath capable of resisting oxidation at high temperature conditions. Heretofore, the art has endeavored to meet these and other requirements by utilizing particular insulating materials in combination with electrical resistance elements and sheaths of de'linite composition so that the resulting eating unit would exhibit a desired combination of properties when subjected to high temperature operation. Typical of the electrical heating units heretofore employed are the well-known units sold commercially under the trademark Calrod which generally comprise a metallic sheath encasing a magnesium oxide insulating medium which, in turn, contains a resistance wire of a nickelchromium alloy such as sold commercially under the trademark Nichrome. One of the principal causes of failure of such sheathed electrical heating units when operated at high temperatures, particularly above about l600 F., has been attributed to the loss of the insulating characteristics of the insulating material, e. g., magnesium oxide. It has been proposed to substitute beryllium oxide or a mixture of beryllium and magnesium oxide for magnesium oxide in order to obtain a heating unit having a substantially constant electrical resistance. Other insulating mediums for divers purposes have been prepared from silicates, the oxides of aluminum, zirconium, thorium, silicon, etc., and various combinations thereof.

It has also been proposed to produce heating units adapted to withstand the demands of high temperature operation by employing sheaths of a particular composition, e. g., nickel-chromium-iron alloys such as the alloys nominally containing approximately 34% nickel, about 21% chromium and about 43% iron sold commercially under the trademark lncoloy or alloys nominally containing about 75% nickel, about 15% chromium and about 6% iron sold commercially under the trademark lnconel. Such alloys insure resistance to oxidation under elevated temperature conditions and also reduce loss in the electrical resistance of insulating materials such as magnesium oxide. Although such heating units have proven acceptable for certain applications, they have been generally limited to normal operating temperatures of up to a maximum of about l600 F. When subjected to temperatures contemplated within the scope of this invention, the aforementioned heating units have performed unsatisfactorily. Moreover, the problem has remained wherein the insulating material, e. g., magnesium oxide, of an electrical heating unit employing conventional heating elements, e. g., made of alloys containing nickel and chromium, suffers a breakdown in its insulating characteristics and, more particularly, in its dielectric strength when subjected to operation at high temperatures above about l600 F.

Although many attempts were made to overcome the foregoing diiculties and other difficulties, none, as far as l am aware, was entirely successful when carried into practice commercially on an industrial scale.

lt has now been discovered that a new and improved sheathed electrical heating unit may be produced by employing therein a specially-prepared electrical resistance element which enables the heating unit to substantially retain its operating characteristics at temperatures above about l600 F. for periods of time not attainable by the use of conventional heating units.

lt is an object of this invention to provide a new and improved electrical heating unit capable of operating for long periods of time at high temperatures.

Another object of the invention is to provide a new and improved electrical heating unit capable of operating in the temperature range of about l600 F. to about 2200 F. for relatively or materially longer periods of time than heretofore possible when using conventional electrical heating units.

This invention also contemplates providing an electrical heating unit in which the insulating and dielectric characteristics of a refractory insulating material are not substantially impaired by high temperature operation.

It is another object of this invention to provide a new and novel process for attaining the foregoing objects.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawing in which:

Figure l is a fragmentary view of a sectional elevation of an electrical heating unit contemplated within the scope of the present invention;

Fig. 2 depicts a sectional view along line 2 2 of Figure l; and

Fig, 3 is an enlarged view, partly in section, of a composite electrical resistance element in accordance with the instant invention.

Broadly stated, the present invention contemplates a new and improved sheathed electrical heating unit in which the surface of a base metal electrical resistance element made of an alloy containing iron group metal and at least one element from the group consisting of chromium and molybdenum is provided with a uniform and rmly bonded layer of nickel, which, in combination with the other components of the sheathed electrical heating unit, i. e., a metallic sheath and insulating material, enables the attainment of much improved service life as compared to units devoid of said nickel layer.

ln carrying the present invention into practice, the nickel layer is metallurgically alloy-bonded to the base metal electrical resistance element or conductor and has a thickness of about 0.0001 inch to approximately 0.05 inch, preferably about 0.00025 inch to about 0.005 inch. The aforesaid electrical resistance element can comprise any suitable metallic material which can be generally characterized as exhibiting high electrical resistance properties and an ability to generate high conversion of electrical energy to heat when an electric current is passed therethrough. The resistance element, which is embedded in a heat-conducting and electrically insulating refractory material, must, of course, have a relatively high melting point to withstand the relatively high temperatures developed. The insulating material and resistance element are enclosed within an oxidation-resistant metallic sheath.

Typical allo-ys that may be used in the present invention as resistance elements are exemplified by the nickelchromium alloys, including alloys containing iron, chromium and nickel, which generally comprise about to 90% nickel, about 10% to 35% chromium, and up to about 65% iron. Other electrical resistance elements which can be satisfactorily employed in accordance with the present invention include nickel-molybdenum alloys, nickel-molybdenum-chromium alloys, nickelcobalt-chromium alloys, iron-chromium alloys, ironchromium-aluminum alloys, and cohalt-chrornium-iron alloys. The foregoing alloys, in addition to containing about 10% to 35%, preferably about 15% to 30%, of metal from the group consisting of chromium and molybdenum, may optionally include small amounts up to a total of about 20%, e. g., 0.01% to 20% of metal from the group comprising tungsten, columbium, zirconium, titanium, silicon, manganese, aluminum, magnesium, etc. The balance of the alloy is comprised essentially of one or more of the iron group metals, i. e., a metal from the group consisting of nickel, iron and cobalt, in a total amount constituting at least about 65% of the alloy. Although the iron group metal content generally exceeds about 65 of the total weight of the alloy, the present invention includes those alloys wherein the individual iron group metals can exist in amounts less than 65% but the sum of which is in excess of about 65 Small amounts of other ingredients may be present as incidental elements or impurities, e. g., calcium, rare earth metals, sulfur, phosphorus, etc. Alloys containing about 20% chromium, balance essentially nickel; alloys containing about chromium, 20% iron, balance essentially nickel; and alloys containing about 15% to 30% molybdenum, with or without about 10% to 25% chromium, balance essentially nickel are preferred electrical resistance element alloys. The resistance element can assume any desired configuration or cross section, such as wire, strip, helix, etc.

The nickel can be applied to the surface of the electrical heating element in any suitable well-known manner provided the nickel is firmly and uniformly bonded to the heating element surface. It is preferred to deposit the nickel by hot-working procedures or electrolytically although other satisfactory methods may be employed, such as vapor deposition, metal spraying, overlaying, etc. Satisfactory nickel-clad heating elements may be obtained by utilizing the hot-working procedures for producing nickel-clad products as disclosed in the U. S. Patents Nos. 2,037,732 and 2,037,733 to W. A. Mudge. The nickel layer should contain at least about 93%, preferably at least about 99% nickel. Other elements which may be present are manganese, silicon, aluminum, titanium and cobalt. It is an essential requirement of this invention that the nickel and the electrical resistance element form a lirm bond which is resistant to cracking, peeling, etc., under heat, applied stresses, etc. Once the bond is destroyed or if no bond is produced, the effectiveness of the heating unit is impaired.

The metallic sheath can comprise any material generally exhibiting substantial resistance to oxidation at temperatures up to about 2200 F. Nickel-chromium alloys, including nickel-chromium-iron alloys, etc., perform satisfactorily as sheaths. Generally, satisfactory sheath materials contain about 10% to 75% nickel, about 15% to 30% chromium, with the balance essentially iron but not excluding small amounts of minor elements such as manganese, silicon, carbon, magnesium, aluminum, titanium, etc., and normal impurities associated therewith. The following table is illustrative of typical alloys which may be suitably employed within the scope of the in vention as sheathing materials.

lMinor elements such as manganesc, silicon, carbon, magnesium, calcium, aluminum, titanium, etc., may be included. The inner surface of the sheath can benecially be nickel.

The insulating material may comprise any suitable material which is substantially electrically non-conducting at high temperatures while exhibiting high heat conductivity, good dielectric strength and high refractory properties, and which does not fuse with the heating element and/0r the sheath material. Suitable materials well known in the art, such as the oxides of magnesium, beryllium, thorium, zirconium, aluminum, titanium, or various mixtures thereof, etc., may be used. It is preferred to use magnesium oxide either alone or in combination with at least one other non-conducting material. The insulating material may assume a powdered, granular, comminuted, etc., form and may be compacted to any desired degree.

For the purpose of giving those skilled in the art a better understanding of the invention, reference is now made to the drawings in describing an illustrative electrical heating unit produced in accordance with the concepts of the instant invention.

In Figures 1 and 2, the electrical heating unit generally comprises a spiral heat-resistant element 1 made of a nickel-chromium alloy, the surface of which is provided with a firmly bonded and uniform cladding or coating 2 of nickel (Fig. 3). The heat-resistant element is supported and positioned approximately centrally and in parallel relation to a nickel-chromium-iron alloy sheath 3 by a powdered mass of magnesium oxide insulating material 4. The respective ends of the heat-resistant alloy 1 may be connected terminals (not shown) in any suitable manner well known to those skilled in the art.

It is necessary, as stated hereinbefore, to obtain a firm and uniform bond between the electrical resistance or heating element and the nickel. The nickel coating described in connection with Figures 1 to 3 was obtained in the following manner. The nickel-chromium resistance wire, approximately 0.235 inch in diameter, was degreased with carbon tetrachloride, scrubbed with wet pumice and then rinsed with demineralized water. The resistance wire was cleaned cathodically for about 2 minutes at a current density of about 50 amperes per square foot in an alkaline solution containing about 6% sodium carbonate. After rinsing with demineralized water, the wire was then irnmersed in an aqueous acid nickel chloride bath containing about 250 grams per liter of nickel chloride hexahydrate and approximately 36.5 gram per liter of hydrochloric acid. The bath was maintained at a temperature of approximately F. At a current density of about 60 amperes per square foot, the resistance wire was treated anodically for approximately 2 minutes and then cathodically for about 6 minutes. This treatment is beneficial in securing a strong adherent coating of nickel on the surface of the resistance wire. The nickel-chromium Wire was then transferred to a Watts-type nickel plating bath where it was plated for about 5 minutes at a current density of about 5 amperes per square foot. The current density was then increased to about 50 amperes per square foot and the plating was continued until a desired nickel plate thickness of about 0.010 inch, as measured on the radius, was obtained. The temperature of the plating bath was maintained at approximately 140 F. and the pH at about 2.0. It was observed that there was a firm mechanical bond between the nickel plate and the nickelchromium resistance Wire. The resulting composite product was then given a series of annealing and drawing steps until a metallurgically alloyed-bond and a predetermined dimension for the composite product was obtained, i. e., at a final diameter of about 0.011 inch, the thickness of the nickel was approximately 0.00075 inch, as measured on the radius. It is important, in accordance with the concepts of the instant invention, to control the thickness of the nickel cladding. The thickness may vary from about 0.0001 inch to about 0.05 inch and it is preferred to operate at the lower side of a range of about 0.00025 inch to about 0.005 inch.

The present invention, in addition to providing the advantages described hereinbefore, also serves to provide new and improved heating units which are capable of securing more rapid and efficient heating. Thus, since thc heating units produced in accordance with the concepts of the instant invention can operate at temperatures of about 1600" F. to about 2200 F., greater amounts of heat may be transferred in short periods of time from the heating unit to the device, medium, material, element, etc., which is to be heated.

The present invention is applicable to sheathed-type electric heating units in general and, more specifically to those heating units having a base metal electrical resistance element made of an alloy containing an iron group metal and at least one element from the group consisting of chromium and molybdenum. Such heating units may be used for a variety of purposes, particularly where temperatures above about l600 F. are encountered, e. g., electric furnaces, electric ranges, kilns, roasting ovens, etc. Other 4applications wherein such heating units are or may be employed are domestic devices such as irons, toasters, broilers, and the like.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

l claim:

1. A new and improved electrical heating unit adapted to be operated at relatively high temperatures of about 1600 F. to about 2200 F. for long periods of time comprising a heat refractory and electrically insulating material comprised essentially of magnesium oxide and encased in a metallic sheath comprised of a nickel-chromium-iron alloy, and a base metal electrical resistance element embedded in said insulating material and comprised of an alloy containing iron group metal and at least one metal from the group consisting of chromium 'and molybdenum, said resistance element having a metallurgically alloy-bonded surface layer of nickel.

2. As a new article of manufacture, a new and improved electrical heating unit adapted to be operated at relatively high temperatures of about 1600" F. to about 2200" F. for long periods of time comprising a heat refractory and electrically insulating material containing at least one oxide selected from the group consisting of magnesium, beryllium, thorium, zirconium, aluminum and titanium and encased in a metal sheath containing about 10 to 75% nickel, about 15 to 30% chromium, the balance essentially iron, and a base metal electrical resistance element embedded in said insulating material and comprised of an alloy containing iron group metal in an amount of not less than 65% by weight of the alloy, the balance of said alloy being essentially a metal selected from the group consisting of chromium and molybdenum, said resistance element having a firmly and uniformly bonded surface layer of nickel.

3. An article of manufacture as described in claim 2 wherein the metal sheath has a layer of nickel firmly and uniformly bonded to its interior surface.

References Cited in the le of this patent UNITED STATES PATENTS 1,359,400 Lightfoot Nov. 16, 1920 1,367,341 Abbott Feb. 1, 1921 1,478,845 Berry Dec. 25, 1923 1,862,138 Elsey June 7, 1932 1,869,140 Gelinas July 26, 1932 2,085,730 Cox et al. July 6, 1937 2,432,149 Griffiths Dec. 9, 1947 

1.A NEW AND IMPROVED ELECTRICAL HEATING UNIT ADAPTED TO BE OPERATED AT RALATIVELY HIGH TEMPERATURE OF ABOUT 1600* F, TO ABOUT 2200* F. FOR LONG PERIODS OF TIME COMPRISING A HEAT REFRACTORY AND ELECTRUCALLY INSULTING MATERIAL COMPRISES ESSENTIALLY OF MAGNESIUM OXIDE AND EMCASED IN A METALLIC DHEAT COMPRISES OF A NICKEL-CHROMIMUM-IRON ALLOY, AND A BASE METAL ELECTRICAL RESISTANCE ELEMENT EMBEDDED IN SAID INSULATING MATERRIAL AND COMPRISED OF AN ALLOY CONTAINING IRON GROUP METAL AND A LEAST ONE METAL FROM THE GROUP CONSISTING OF CHROMIUM AND MOLYBDEUM, SAID RESISTANCE ELEMENT HAVING A METALLURGICALLY ALLOY-BONDED SURFACE LAYER OF NICKEL. 